The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A bistable electromagnetic actuator can be used to control a mechanism by means of a short duration current/voltage pulse. This type of actuator typically comprises one or more coils, a plunger, a permanent magnet and a casing with two cores, one at either end of the plunger. This actuator enables the plunger to be kept in position against one or the other of the two cores without continuously supplying a control current, thanks to the permanent magnet. Actuators of this type can be used to make rapid changes in position, generate high forces and have long strokes. These features can be combined as a function of the coil and the current supplied to it.
This type of actuator is particularly suitable for the production of valves. An actuator in a cryogenic valve, for example, is subject to high thermal and mechanical loads. Fluid pressure makes great demands on the materials and can lead to breakdown. Temperature differences generated by the fluid cause the material to expand as the temperature can vary from a few degrees kelvin (K.) to over 473° K. When several materials with different coefficients of expansion are in contact, differential expansion adds to the mechanical stress in the materials.
U.S. Pat. No. 5,127,585 discloses a high-pressure fuel injection valve comprising a bistable electromagnetic actuator. The magnetic actuator comprises a needle which is connected to a plunger located in a field generated by two coils and a permanent magnet sandwiched between the coils. Nonmagnetic sleeves are located inside the coils which house the magnetic poles, directing the magnetic flux to the plunger. This type of actuator enables fluid flow to be controlled precisely, allowing the valve to be opened for a short duration while working at high pressure.
U.S. Pat. No. 3,755,766 discloses a bistable electromagnetic actuator. It comprises a cylindrical body in which there is a coil that houses a plunger that is free to move in translation. The actuator further comprises a spring and a permanent magnet arranged axially so as to hold the plunger in position in either one of its two equilibrium positions. A sleeve is mounted in the actuator separating the plunger and the coil. This type of actuator is used to move the plunger from one position to another that is a long distance away, and to keep the plunger in position without supplying any power. This actuator includes a sleeve with low magnetic permeability. The plunger is held in one position by means of a spring, which adds a component and increases the size of the actuator.
U.S. Pat. No. 4,829,947 discloses a bistable electromagnetic valve actuator. The actuator comprises two concentric coils between which there is an annular permanent magnet. A plunger, movable in translation, is mounted in the center of the coils and the permanent magnet. A thin sleeve is fixed to the internal face of the permanent magnet to better distribute the magnet flux and to protect it mechanically.
U.S. Pat. No. 5,144,272 discloses an electromagnetic actuator comprising a coil in which two cores are located and a movable plunger inserted into the cores. The actuator further comprises a sleeve made of magnetic material extending over only a portion of the coil so as to direct the magnetic field towards the plunger.
U.S. Pat. No. 6,903,647 discloses a valve with an electromagnetic actuator which comprises a coil in which a sleeve, consisting of two part-sleeves nested one inside the other is located; one part-sleeve is magnetically permeable, the other not. This feature creates a discontinuity in the magnetic field or air gap.
Generally, for the actuators described above, one or more pieces of nonmagnetic material are usually placed around the air gap between the cores and the plunger to avoid magnetically short circuiting the air gap. Locating this or these piece(s) of nonmagnetic material side by side with the ferromagnetic parts of the sleeve or the casing requires a complex stack of parts which can make its manufacture expensive, particularly when the actuator is to be sealed.